wireless embedded systems (0120442x) medium access control
DESCRIPTION
Wireless Embedded Systems (0120442x) Medium Access Control. Chaiporn Jaikaeo [email protected] Department of Computer Engineering Kasetsart University. Materials taken from lecture slides by Karl and Willig. Overview. Principal options and difficulties Contention-based protocols - PowerPoint PPT PresentationTRANSCRIPT
Network Kernel Architectures
and Implementation(01204423) )
Medium Access Controland WPAN Technologies
Chaiporn [email protected]
Department of Computer EngineeringKasetsart University
Materials taken from lecture slides by Karl and Willig
2
Overview Principal options and difficulties Contention-based protocols Schedule-based protocols Wireless Personal Area Networks
Technologies
3
Difficulties Medium access in wireless networks
is difficult, mainly because of Half-duplex communication High error rates
Requirements As usual: high throughput, low
overhead, low error rates, … Additionally: energy-efficient, handle
switched off devices!
4
Requirements for Energy-Efficient MAC Protocols Recall
Transmissions are costly Receiving about as expensive as transmitting Idling can be cheaper but is still expensive
Energy problems Collisions Overhearing Idle listening Protocol overhead
Always wanted: Low complexity solution
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Main OptionsWireless medium access
Centralized Distributed
Contention-based
Schedule-based
Fixedassignment
Demandassignment
Contention-based
Schedule-based
Fixedassignment
Demandassignment
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Centralized Medium Access A central station controls when a
node may access the medium E.g., Polling, computing TDMA
schedules Advantage: Simple, efficient
Not directly feasible for non-trivial wireless network sizes
But: Can be quite useful when network is somehow divided into smaller groups
Distributed approach still preferable
7
Schedule- vs. Contention-Based Schedule-based protocols
FDMA, TDMA, CDMA Schedule can be fixed or computed on
demand Usually mixed
Collisions, overhearing, idle listening no issues Time synchronization needed
Contention-based protocols Hope: coordination overhead can be saved Mechanisms to handle/reduce
probability/impact of collisions required Randomization used somehow
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Overview Principal options and difficulties Contention-based protocols Schedule-based protocols Wireless Personal Area Networks
Technologies
9
A
Distributed, Contention-Based MAC Basic ideas
Receivers need to tell surrounding nodes to shut up
Listen before talk (CSMA) Suffers from sender not knowing what is
going on at receiver
B C D
Hidden terminal
scenario: Also: recall exposed terminal scenario
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How To Shut Up Senders Inform potential interferers during
reception Cannot use the same channel So use a different one
Busy tone protocol Inform potential interferers before
reception Can use same channel Receiver itself needs to be informed, by
sender, about impending transmission Potential interferers need to be aware of such
information, need to store it
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MACA Multiple Access with
Collision Avoidance Sender B issues
Request to Send (RTS)
Receiver C agrees with Clear to Send (CTS)
Potential interferers learns from RTS/CTS
B sends, C acks Used in IEEE
802.11
A B C D
RTS
CTS
Data
Ack
NAV indicates busy medium
NAV indicates busy medium
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Virtual Carrier Sensing
RTS
CTS
Data
ACK
A B C D
NAVNAV
NAV Network Allocation Vector
(Virtual Carrier Sensing)
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Problems Solved? RTS/CTS helps, but do not solve
hidden/exposed terminal problemsA B C D
RTS
CTS
Data
A B C D
RTS
RTS
CTS
RTS
RTSCTS
CTSData
Data
Ack
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MACA Problem: Idle listening Need to sense carrier for RTS or CTS
packets Simple sleeping will break the protocol
IEEE 802.11 solution Idea: Nodes that have data buffered for
receivers send traffic indicators at prearranged points in time
ATIM - Announcement Traffic Indication Message
Receivers need to wake up at these points, but can sleep otherwise
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Sensor-MAC (S-MAC) MACA unsuitable if average data
rate is low Most of the time, nothing happens
Idea: Switch off, ensure that neighboring nodes turn on simultaneously to allow packet exchange Need to also exchange
wakeup schedule between neighbors
When awake, perform RTS/CTS
Wakeup period
Active period
Sleep period
For SYNCH For RTS For CTS
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Listen for SYNC
td
Schedule Assignment Synchronizer
Listen for a mount of time If hear no SYNC, select its own SYNC Broadcasts its SYNC immediately
Follower Listen for amount of time Hear SYNC from A, follow A’s SYNC Rebroadcasts SYNC after
random delay td
Sleep
Listen
Go to sleep after time t
Sleep
Listen
Broadcasts
A
B
Broadcasts
Go to sleep after time t- td
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S-MAC Synchronized Islands Nodes learn schedule from other nodes Some node might learn about two
different schedules from different nodes “Synchronized islands”
To bridge this gap, it has to follow both schemes
Time
A A A A
C C C C
A
B B B B
D D D
A
C
B
D
E E E EE E E
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Preamble Sampling Alternative option: Don’t try to explicitly
synchronize nodes Have receiver sleep and only periodically
sample the channel Use long preambles to ensure that
receiver stays awake to catch actual packet Example: B-MAC, WiseMAC
Check channel
Check channel
Check channel
Check channel
Start transmission:Long preamble Actual packet
Stay awake!
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B-MAC Very simple MAC protocol Employs
Clear Channel Assessment (CCA) and backoffs for channel arbitration
Link-layer acknowledgement for reliability
Low-power listening (LPL) I.e., preamble sampling
Currently: Often considered as the default WSN MAC protocol
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B-MAC B-MAC does not have
Synchronization RTS/CTS Results in simpler, leaner
implementation Clean and simple interface
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Clear Channel Assessment "Carrier Sensing" in wireless
networks
Thresholding CCA algorithm
Outlier detection CCA algorithm
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Contiki LPL and LPP Low-Power Listening (LPL)
Also known as ContikiMAC Similar to B-MAC, but allowing packet-
based MAC such as IEEE 802.15.4 Low-Power Probing (LPP)
Receivers periodically broadcast a probe
Sender listens for probes from receivers before transmitting
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Overview Principal options and difficulties Contention-based protocols Schedule-based protocols Wireless Personal Area Networks
Technologies
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LEACH Low-Energy Adaptive Clustering
Hierarchy Assumptions
Dense network of nodes Direct communication with central sink Time synchronization
Idea: Group nodes into “clusters” Each cluster controlled by clusterhead About 5% of nodes become clusterhead
(depends on scenario) Role of clusterhead is rotated
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LEACH Clusterhead Each CH organizes
CDMA code for its cluster TDMA schedule to be used within a
cluster In steady state operation
CHs collect & aggregate data from all cluster members
Report aggregated data to sink using CDMA
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LEACH rounds
Setup phase Steady-state phase
Fixed-length round
……….. ………..
Advertisement phase Cluster setup phase Broadcast schedule
Time slot 1
Time slot 2
Time slot n
Time slot 1…..….. …..
Clusterheads compete with CSMA
Members compete with CSMASelf-election of
clusterheads
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TRAMA Traffic Adaptive Medium Access
Protocol Assume nodes are time synchronized Time divided into cycles, divided into
Random access period Scheduled access period
Random Access Period Scheduled-Access Period
time cycle
• Exchange and learn two-hop neighbors
• Exchange schedules
• Used by winning nodes to transmit data
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TRAMA – Adaptive Election How to decide which slot (in scheduled
access period) a node can use? For node id x and time slot t, compute p = h (x
t) h is a global hash function
Compute p for next k time slots for itself and all two-hop neighbors
Node uses those time slots for which it has the highest priority
t = 0
t = 1
t = 2
t=3 t = 4
t = 5
A 14 23 9 56 3 26B 33 64 8 12 44 6C 53 18 6 33 57 2
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Overview Principal options and difficulties Contention-based protocols Schedule-based protocols Wireless Personal Area Networks
Technologies
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IEEE 802.15.4 IEEE standard for low-rate WPAN (LR-WPAN)
applications Low-to-medium bit rates Moderate delays without too strict requirements Low energy consumption
Physical layer 20 kbps over 1 channel @ 868-868.6 MHz 40 kbps over 10 channels @ 905 – 928 MHz 250 kbps over 16 channels @ 2.4 GHz
MAC protocol Single channel at any one time Combines contention-based and schedule-based
schemes Asymmetric: nodes can assume different roles
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868MHz / 915MHz PHY
2.4 GHz
868.3 MHz
Channel 0 Channels 1-10
Channels 11-26
2.4835 GHz
928 MHz902 MHz
5 MHz
2 MHz
2.4 GHz PHY
802.15.4 PHY Overview Operating frequency bands
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802.15.4 Device Classes Full function device (FFD)
Any topology Network coordinator capable Talks to any other device
Reduced function device (RFD) Limited to star topology Cannot become a network coordinator Talks only to a network coordinator Very simple implementation
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802.15.4 Network Topologies
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802.15.4 Beaconed Mode Superframe structure
GTS assigned to devices upon request
Active period Inactive period
Contention access period
Guaranteed time slots (GTS)
Beacon
802.15.4 GTS Data Transfer Device coordinator
If having allocated GTS, wake up and send
Otherwise, send during CAP Using slotted CSMA
Coordinator device If having allocated GTS,
wake up and receive Otherwise, see picture
Coordinator Device
Beacon
Data request
Acknowledgement
Data
Acknowledgement
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IEEE 802.15.4 Adopters ZigBee
Requires battery life of at least two years be certified
Applications: Industrial control, embedded sensing, home automation
ZigBee RF4CE (Radio Frequency for Consumer Electronics)
Nest (acquired by Google) Learning thermostats,
Smoke and CO alarms WiFi- and ZigBee-enabledhttps://nest.com
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Bluetooth Smart Formally Bluetooth Low Energy (BLE)
Part of Bluetooth 4.0 Specification Based on Nokia's Wibree technology First smartphones to support
iPhone 4S Now supported by most recent
smartphones
http://redbearlab.com/blenano/
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Bluetooth: Classic vs. Smart
Source: Bluetooth SIG
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Bluetooth Compatibility
http://blog.laptopmag.com/just-what-is-bluetooth-4-0-anyway
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Bluetooth Smart: Device Roles Central device
Serves as a hub to one or more peripheral devices
Two central devices cannot directly communicate
Similar to IEEE 802.15.4's FFD Peripheral device
Must be connected to a central device Two peripheral devices cannot directly
communicate Similar to IEEE 802.15.4's RFD
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ANT / ANT+ / NIKE+ Primarily used for
fitness monitoring devices
ANT / ANT+ open access multicast
wireless sensor network
NIKE+ Proprietary protocols
on 2.4 GHz band
http://developer.sonymobile.com
Nike.com
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WiFi/ZigBee/Bluetooth Coexistence They all employ 2.4 GHz spectrum
http://www.digikey.com/en/articles/techzone/2011/aug/comparing-low-power-wireless-technologies
WiFi vs. Zigbee WiFi vs. Bluetooth
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Summary Many different ideas exist for medium access
control in MANET/WSN Comparing their performance and suitability is
difficult Especially, clearly identifying interdependencies
between MAC protocol and other layers/applications is difficult Which is the best MAC for which application?
Nonetheless, certain “common use cases” exist IEEE 802.11 DCF for MANET IEEE 802.15.4 for some early “commercial” WSN
variants B-MAC for WSN research not focusing on MAC